The present invention relates to a room temperature curable silicone rubber composition or, more particularly, to a room temperature curable organopolysiloxane rubber composition with good curability capable of giving a cured rubbery elastomer having an outstandingly good heat resistance and small permanent compression set.
There are known several types of room temperature curable silicone rubber compositions curable with different mechanisms of crosslink formation, of which the most widely used one is the composition curable by the condensation reaction between silanolic hydroxy groups at both of the molecular chain ends of a diorganopolysiloxane and alkoxy groups in an organosilane or organopolysiloxane compound having at least three of such alkoxy groups in a molecule in the presence of a curing catalyst. Conventional catalysts effective to such a condensation reaction are organotin compounds such as dibutyltin dilaurate and dibutyltin dioctoate.
A problem in these room temperature curable silicone rubber compositions is the poor mechanical properties of the cured rubbery elastomers thereof, in particular, in respect of the permanent compression set. That is, when a cured rubbery elastomer body was kept under compressive force for a long time, the initial uncompressed form of the elastomer body can be regained only partly even by the release of the outer compressive force and the permanent compression set, i.e. the percentage of the unregained compression of the elastomer body, is remarkably larger in the cured elastomer body of the room temperature-curable silicone rubber compositions than of the heat-curable silicone rubber compositions. Such a disadvantageous property of the compositions greatly limits the applicability of the composition to practical uses. For example, gaskets made of such a room temperature-curable silicone rubber composition are not free from the decrease in the packing effect when the gasket is used for a long period of time under compression. When a rubber roller of such a room temperature-curable silicone rubber on a core mandrel of metal is run after a long period of time under a contacting pressure with another roller, the silicone rubber roller has been deformed and the cross section thereof is no longer exactly circular so that the revolution of the roller is accompanied by the generation of noises or the design pressure cannot be obtained at the deformed portion. Such a problem in the rubber roller formed of a room temperature-curable silicone rubber can be overcome only by the use of an elaborate mechanical means with which the rubber portion of the roller is not under compression when the roller is not in running.
Owing to the above described disadvantageous property of the large permanent compression set, room temperature-curable silicone rubber compositions are excluded from the industrial applications in which the rubber parts are under continuous compression prolongedly despite the advantageous properties in comparison with heat-curable silicone rubbers in respect of the workability, adhesiveness to the substrate surface and mold-releasability as well as the short half-life for the decay of the static electricity accumulated thereon. Therefore, one of the most important problems for investigations in the technology of room temperature-curable silicone rubber compositions has been the improvement in respect of the permanent compression set.
Various attempts and proposals have of course been made in this regard. For example, decrease in the amount of the organotin compound as the curing catalyst is effective to some extent though with a great sacrifice in the curing velocity so that such a silicone rubber composition requiring an unduly long curing time is industrially no longer practical even by the extended pot life. The use of tin (II) octoate as the curing catalyst in place of the above named organotin compounds is indeed effective to decrease the permanent compression set but the catalytic activity of this compound is very large and too sensitive to the amount of formulation so that difficulties are encountered in obtaining an accurate and reproducible formulation of the composition if not to mention the disadvantages for the industrial application due to the remarkably decreased pot life of the catalyst-blended composition and the inherent instability of this compound against humidity. Another problem in the room temperature-curable silicone rubber composition crosslinkable by the condensation reaction is the relatively low heat resistance of the cured rubbery elastomer in comparison with those curable by the addition reaction between vinyl groups bonded to the silicon atoms and the hydrogen atoms directly bonded to the silicon atoms in the presence of a platinum catalyst. For example, the hardness of the cured rubber of the condensation-type room temperature-curable silicone rubber composition may be greatly decreased when the rubber is heated at 200.degree. C. or above or kept in a hermetically sealed condition at 150.degree. C. or above. Further, the reliability of curing is somewhat low in the condensation-type room temperature-curable silicone rubber compositions when a rubber article of a large wall thickness is desired resulting in a considerably large difference between the values of hardness at the surface layer and in the depth of the thick wall.